Traveling wave tubes



April 2, 1963 E. c. DENCH ,084,

TRAVELING WAVE TUBES Filed April 30, 1958 2 Sheets-Sheet l /A vs' 702 Eawaeo c Arm/ems! a Q y i U Maw- United States Patent 3,084,277 TRAVELING WAVE TUBES Edward C. Dench, Needham, Mass, assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filer] Apr. 3a, 1958, Ser. No. 733,228 7 Claims. cl. 315-39.

This invention relates to a traveling wave tube, and more specifically, to a traveling wave tube wherein one of the interaction space-bounding electrodes is so shaped that the interaction space varies across the transverse dimension thereof.

Traveling wave tubes are known which include a slow wave energy propagating structure or delay line, a continuous electrode or sole spaced from and disposed substantially parallel to said structure, and an electron gun mounted adjacent one end of said structure for producing an electron beam which, under the influence of an electric field existing between said structure and said sole and a magnetic field transverse to the electric field and to the beam, traverses the interaction space bounded by the slow wave structure and the sole. Such traveling wave tubes are widely used either as amplifiers capable of operation over a large band width or as oscillators capable of being tuned electronically over a considerable frequency range in the microwave region. Such devices utilize the interaction between an electron beam moving along paths adjacent a periodic non-resonant slow wave propagating structure and the electromagnetic field of the radio frequency wave propagating along said periodic structure. The electromagnetic field along such a periodic structure may be resolved into a number of superimposed traveling waves or space harmonics each having its own phase velocity. Some of the space harmonics of a phase velocity travel in the same direction as the wave energy or group velocity and are referred to as forward waves. Other space harmonics, on the other hand, have a phase velocity of opposite sense, that is, the phase velocity is in a direction opposite to the energy or group velocity. Such harmonics are referred to as backward waves. If the electron beam velocity is adjusted so that it is in substantial synchronism with the phase velocity of a given space harmonic, interaction between the electron beam and this component will occur, and energy will be transferred from the electron beam to the electr0- magnetic field. In a traveling wave amplifier, a radio frequency input signal is coupled to the periodic structure adjacent one end thereof, and, owing to the interaction between electron beams moving along a path adjacent the periodic structure and the electromagnetic field of the radio frequency wave propagating along said structure, amplification of the input signal may be obtained, under proper operating conditions. This amplified signal may then be extracted from the periodic structure adjacent the other (output) end thereof. In the forward wave amplifier the interaction is between the electron beam and a forward wave; the electrons thus are projected toward the output end of the periodic structure. In the backward wave amplifier, interaction occurs between the electron beam and a backward wave and the electrons then move toward the input end of the periodic structure. In a traveling Wave oscillator, when the electron beam current exceeds a critical current at which oscillations can begin, and when the electron beam velocity is substantially equal to the velocity of a suitable space harmonic of the propagated wave, oscillations may be generated within the device and the generated energy will propagate along the periodic structure and may be extracted at one end thereof.

The average velocity of the electron beam is equal to the ratio of the intensity of the electric field between "ice the delay line and the sole to the strength of the transverse magnetic field. The electric field strength, in turn, is inversely proportional to the spacing of the delay line and the sole.

A traveling wave tube of the backward wave type is discussed in United States Letters Patent of Edward C. Dench and Albert D. La Rue, Patent No. 2,888,649 issued May 26, 1959, and includes a nonreentrant slowwave energy-propagating structure or delay line, a continuous electrode or sole spaced from and disposed substantially parallel to said structure, and an electron gun mounted adjacent one end of said structure for producing an electron beam which, under the influence of an electric field existing between said structure and said sole and a magnetic field transverse to said electric field, traverses the interaction space bounded by the slow-wave structure and the sole.

Because of the interaction between the electron beam and high frequency fields of wave energy propagating along the slow-wave structure, energy may be transferred from the electron beam to the high frequency field. An R.F. signal introduced into one end of said structure may be amplified in this manner, or, the tube may be made to oscillate as the result of this energy exchange.

Because of practical manufacturing conditions, it is often difiicult to maintain constant the spacing between the delay structure and the sole along the interaction space. It has been found, for example, that in a tube having a sole-delay structure spacing of 0.10 inch, a variation in this spacing of as little as 0.001 inch represents a one percent variation in electron beam velocity, since, as previously mentioned, the average electron beam velocity of such a tube is equal to the ratio of the electric field strength to the magnetic field strength, and since the electric field strength is directly dependent upon the spacing between the delay structure and the sole. In order to render the traveling wave tube less sensitive to sole-delay structure spacing, the sole surface along the transverse dimension of the sole and the interaction space, that is, the dimension perpendicular both to the direction of propagation of the electron beam and to the electric field between the delay structure and the sole, is made variable so that there is a slight variation of the electric field across any transverse section of the interaction space. By transverse section is meant a section disposed substantialiy perpendicular to the direction of propagation of the electron beam and perpendicular to the electric fieid between the delay structure and the sole. Experiments with a traveling wave oscillator using a sole of this type have indicated that the power output is less sensitive to change in voltage between the sole and the cathode over a predetermined frequency band than a traveling wave oscillator using the conventional sole of rectangular configuration.

The sole electrode is arranged generally parallel to the delay structure, in the case of a traveling wave tube of linear configuration, and generally concentric with said delay structure, in the case of a circular traveling wave tube, and usually includes vertical walls or side members at each edge of the peripheral portion of the sole facing the electron beam which approach or may even slightly overlap the delay structure. These vertical walls serve as a beam-forming end shield to keep the electron beam centered in the system and from diverging in a direction normal to the main path of the electrodes as the result of mutual repulsion of the electrodes in the space charge existing along the length of the tube. In some instances, these side members may be omitted.

In one example of the invention, the peripheral portion of the sole exposed to the electron beam-which, in the case of soles having side members, is the portion of the sole disposed between the side members-is curved so that the spacing between the delay structure and the sole progressively increases as one approaches the center of the transverse section, or thickness dimension, of the sole from the edges of said sole. With this arrangement, the continuous electric field between the delay s ructur and sole tends to focus the beam toward the more active central portion of the delay structure. The amount of variation of the cross-section of the sole is adjusted empirically for optimum operation. It should be understood, however, that the variation should not be so great as to change appreciably the average velocity of the electrons in the interaction space.

Another advantage of the tube according to the invention is that any slight off-centering of the sole, or any taper in sole-delay structure spacing of the tube of the present invention, will be compensated, at least partially, by the fact that at all points along the active length of the tube there will be some part of a cross section having the same electric field as any other cross section. The active portion of the sole, in addition to being continuously curved, may be similar to the conventional U- shaped sole except for rounded fillets in the corner regions adjoining the active surface and the two vertical side members. An alternative to the above arrangements may be a sole active surface consisting of two or more linearly varying portions across the transverse dimension of the sole; for example, the sole surface may be V-shaped, or may consist of several interconnected regions of varying slope, as the periphery of a polygon.

In some instances, it is desirable that the sole be grooved so that secondary electrons resulting from impingement of primary electrons of unfavorable phase traveling toward the sole are caught within the grooves and prevented from reentering the interaction space owing to the shielding effect of the walls of the grooves. In this type of line the various ridges of the sole cross section may be made of variable length; in other words, the surface of the ridges lies along either tapered straight lines or a curve.

Other objects and features of this invention will be understood more clearly and fully from the following detailed description of the invention with reference to the accompanying drawing wherein:

FIG. 1 is a cross-sectional view, partly in elevation, of a traveling wave tube according to the invention;

FIG. 2 is a cross-sectional view, partly in elevation, of the traveling wave tube of FIG. 1 taken along the line 2--2;

FIGS. 3 to 8 are detail views showing various configurations of the sole electrode of the tube of FIGS. 1 and 2;

FIG. 9 is a schematic view of a linear traveling Wave tube;

FIG. 10 is a transverse sectional view of the sole electrode of the tube of FIG. 9 taken along line 1010; and

FIG. 11 is a fragmentary sectional view of an amplifier version of the tube of FIGS. 1 and 2.

Referring to the drawing, a traveling wave oscillator tube 10 is shown which comprises a slow wave energypropagating structure or delay line 12, a cylindrical electrode 14, otherwise referred to as a sole, concentric with the delay line 12 and normally maintained negative with respect thereto, a lead-in assembly 15, an electron gun assembly including at least a cathode 21 and heater 22, a magnetic field-producing means 25 and an output coupling means 17. The circular delay line 12 includes several interdigital fingers or elements 31 and 32 extending from respective oppositely disposed annular members 33 and 34. Members 33 and 34 are secured by screws 35, visible in FIG. 1, to the shoulder portion of a cylindrical electrically-conductive ring 36, said ring forming a part of an evacuated envelope for tube 10. The remainder of the delay line 12 includes a pair of oppositely located cover plates 38 and 39 hermetically sealed to ring 36. Although the delay line has been de- The sole 14 consists essentially of a cylindrical block constructed of an electrically-conductive material and including a web portion bounded by an arcuate section 46 whose periphery consists of an active surface 47 and side or edge members 48. The purpose of these side members 48 is to confine the electron beam within the interaction space 50 between surface 47 of sole 14 and the interdigital line 12. The surface 47 of sole 14 between the side members 48 is curvilinear, as is more clearly apparent in the enlarged fragmentary view of sole 14 of FIG. 3. Other embodiments of sole 14 are shown in FIGS. 4 to 8, inclusive. The sole 14 of FIG. 4 includes the usual side or edge members 48 and a substantially planar central portion 47 joined to the edge members 48 by arcuate corner portions 49. The arrangement of FIG. 4, while not as effective as that shown in FIGS. 1 and 3, has provided increased operating cf ficiency as compared to a sole having square corners, such as that shown in Dench Patent No. 2,809,328, issued October 8, 1957. For example, in one tube wherein the curvature at each edge covered approximately 15 percent of the width of the interaction space defined by the inside dimensions between the end shield edges, the tube operated at 36 percent efliciency, whereas the same tube using a sole with square corners operated at approximately 30 percent efficiency. Another arrangement of sole 14 is shown in FIG. 5 and includes a V-shaped surface 47 in the active region between the edge member 48. This arrangement is more readily machinable than the version shown in FIGS. 3 and 4 and offers advantages of the order of magnitude of those obtainable in the sole of FIG. 3. In FIG. 6, a grooved sole 14 is shown which includes a plurality of longitudinal grooves disposed between adjacent ridges 51. The transverse sole-to-anode spacing of tubes using this type of sole is altered by varying the height of the ridges 51. The ridges may be curved, as shown in FIG. 6, so that the locus of the tips of the ridges follow a smooth curve, or the ridges may be machined in such a way that the locus of the ridge tips lie along a V-shaped path, as in FIG. 5, or along paths similar to those shown in FIGS. 4, 7, and 8. The sole of FIG. 7 is similar to that of FIG. 3, except that the lateral extremities of the curved surface 47 coincide with the free extremities of edge portions 48. In FIG. 8, a sole 14 is shown whose surface 47 comprises a plurality of adjoining planar surfaces, each of different slope. This composite surface 47 of FIG. 8 may be considered as a compromise between a curvilinear surface, such as in FIGS. 3 and 7 and the V-shaped surface of FIG. 5. Although the multiple'planar surface 47 of FIG. 8 terminates in the tips of edge portions 48, as in the case of the sole of FIG. 7, the surface 47 of FIG. 8 may, of course, terminate at each edge and at a point spaced from the tips of edge members 48, as in FIGS. 3 and 5. This is also true of the V-shaped surface of FIG. 5.

A tubular metallic insert 52 is brazed into position against the inner periphery of a centrally disposed aperture 53 in the web portion 45 of sole 14. One end of F a hollow supporting member 54 is inserted within insert 52 and secured firmly thereto. Supporting member 54, in addition to providing support for sole 14, forms a portion of lead-in assembly 15 and allows for passage of external circuit connecting leads, in a manner to be described subsequently.

Sole 14 contains a slot 27 to accommodate the electron gun assembly 20. Since the invention does not involve details of the electron gun 20, the latter is shown schematically in FIGS. 1, 2, and 9. The construction and manner of mounting of the electron gun 20 may be as shown in United States Letters Patent of Roy A. Paananen, Patent No. 2,914,700 issued November 24, 1959. Electron gun 20 includes a cathode 21, a heater 22, a grid electrode 23 which may be used for control of beam current (as for amplitude modulation), and an accelerating electrode 24 which likewise may be used for control of beam current. The cathode 21 may be in the form of a rectangular prism provided with a circular bore in which a heater wire 22 is inserted and electrically insulated from cathode 21; the heater may be connected at one end to the cathode. Electrical energy from appropriate sources is supplied to the cathode 21, heater 22, grid electrode 23 and accelerating electrode 24 by way of respective lead-in wires 61, 62, 63, and 64, which are brought out from the tube envelope through the lead-1n assembly 15. In FIG. 1, the cathode only of the electron gun 20 is shown, for the sake of simplicity and clarity, the remaining elements of the electron gun bemg omitted.

Lead-in assembly 15 includes an electrically-conductive sleeve 66 aflixed to the inner periphery of cover plate 38, as indicated in FIG. 1. A section of cylindrical glass tubing 67 interconnects sleeve 66 and a second electrically-conductive sleeve 68. The other end of tube 68 is provided with a glass seal 69 for sealing the traveling wave tube after evacuation. One end of sole-supporting member 54 contains an outwardly flared portion which is connected to the inner surface 01 sleeve 68. The leads 61, 62, 63, and 64 are mounted in electrically-insulated relation with supporting member 54 by one or more glass beads 71. D

The coaxial output coupling means 17 is sealed in an opening of wall 36 of delay line 12 and is impedancematched to the delay line. The inner conductor 73 of the output coupling means 17 is connected to a finger of delay line 12 at or near the end of the delay line adjacent electron gun 20.

Traveling Wave tube 10 may be provided with a collector electrode 75, shown in FIG. 2, for intercepting electrons after one traversal of the arcuate interaction space 50. This collector electrode may be in the form of a projection from back Wall 36 of delay structure 12. In some instances, however, the collector electrode may be omitted and the electron stream made reentrant.

The necessary electric field between the slow-wave structure 12 and sole 14 may be obtained by means of a unidirectional voltage applied therebetween; such a voltage may be supplied by a battery 83. The sole 14 may be biased negatively relative to the cathode 21 by means of a source 81 of voltage connected between cathode lead 61 and sole-supporting member 54 by Way of sleeve 68. The cathode 21 may in some instances, however, be at the same potential as sole 14; in this case, a source 81 would be omitted. Similarly, the delay line 12 may be maintained at a potential positive relative to both sole 14 and cathode 21 by means of the source 83 of unidirectional voltage connected between the cathode and sleeve 66, the latter being connected, in turn, to delay line 12. The accelerating electrode 24 may be maintained at a potential positive relative to cathode 21 by means of a source 85 of unidirectional voltage connected between leads 61 and 64. The control grid lead 63 may be connected by way of terminal 88 to an appropriate energy source for controlling the magnitude of the electron beam current in the traveling Wave oscillator 10. A suitable heater voltage from a source 82 is connected between heater lead 62 and cathode lead 61.

A uniform magnetic field transverse to the direction of propagation of the electron beam is provided by a permanent magnet or electromagnet having cylindrical pole pieces 91 and 92 radially positioned on or adjacent the anode cover plates 38 and 39, respectively. Pole piece 91 is apertured to receive lead-in assembly 15, While pole piece 92 is apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 50 between sole 14 and delay line 12. By a proper adjustment of the magnitude and polarity of the magnetic and electric fields so established, the electron beam may be made to move along a more or less circular path along the interaction space 50 under the combined influence of these transversely disposed fields.

Although the tube 10 which has been described may be used as an oscillator, the invention also is applicable to a traveling Wave amplifier, which structurally may be the same as the oscillator 10 of FIGS. 1 and 2 except that an additional energy coupling means 19 is provided at the end of the delay structure 12 opposite the output coupling means 17. Such a tube is indicated in fragmentary form in FIG. 11 in 'which elements corresponding to those of the tubes of FIGS. 1 and 2 are indicated by like reference numerals. With the connections as shown in FIG. 10, this tube is adapted to operate as a backward wave amplifier; by reversing the input and output coupling means, the tube could be adapted for operation as a forward Wave amplifier.

The invention may also be incorporated in nonreenform in FIG. 11 in which elements corresponding to those FIGS. 9 and 10. Elements of the tube of FIGS. 9 and it) corresponding to those of FIGS. 1 and 2 are shown by the same reference numerals. The tube of FIG. 9 is constructed so that its fundamental mode of wave propagation is a backward Wave. The tube envelope 11 may be constructed of an electrically-conductive material and may be provided with ceramic or similar electrically-insulating bushings 13 through which electrical connections can be made to the various tube electrodes. An output coupling means 17 is connected to the delay structure 12 adjacent the electron gun 20. The sole electrode 14 is arranged substantially parallel to the delay structure 12. A typical cross-sectional configuration for sole 14 of the tube of FIG. 9 is shown in FIG. 10. It should be understood that any of the configurations shown in FIGS. 3 to 8 maybe used in the tube of FIG. 9, except, of course, that the sole is linear rather than circular. A unidirectional voltage source 83, cooperating with a sole bias source 86, establishes an electric field in the interaction space 50 bounded by delay structure 12 and sole 14. A uniform magnetic field B is established transverse to the electric field in the interaction space 50, as by a permanent magnet. A heater voltage and accelerating anode voltage for the electron gun are provided by batteries 87 and '85, respectively. The linear tube of FIGS. 9 and 10, like the circular tube of FIGS. 1 and 2, may be constructed to operate as an amplifier by the addition of an input coupling means.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A traveling wave electron discharge device comprising a slow Wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy, an electrode spaced from and substantially co-extensive with said slow wave structure, and means for directing electrons in a beam along said region in energyexchanging relation with said fields of wave energy, said electrode having a peripheral portion of its thickness dimension facing said slow wave structure, at least part of said peripheral portion of said electrode having a continuously varying surface contour across its thickness dimension resulting in points on said contour towards the ends thereof being progressively closer to said slow wave structure.

2. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy, an electrode spaced from and substantially co-extensive with said slow wave structure, and means for directing electrons in a beam along said region in energy-exchanging relation with said fields of wave energy, said electrode having a peripheral portion across its thickness dimension facing said slow wave structure, at least a part of said peripheral portion being composed of interconnected surfaces of continuously varying slope across its thickness dimension, points on said surfaces toward the edges of said contour being spaced closer to said slow wave structure than points toward the middle of said contour.

3. A traveling wave electron discharge device comprising a slow wave energy propagating structure, an electrode spaced from and substantially co-extensive with said slow wave structure, and means for directing electrons in a beam along a region adjacent said slow wave structure in energy-exchanging relation with fields of electromagnetic wave energy present in said region, said electrode having a peripheral portion and side walls at the edges of said peripheral portion, at least part of said peripheral portion facing said slow wave structure, said peripheral portion having a continuously varying contour over its thickness resulting in points on said contour toward the ends thereof being spaced progressively closer to said propagating structure.

4. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy, an electrode spaced from and substantially co-extensive with said slow wave structure, said electrode having a peripheral portion facing said slow wave structure, said electrode further having a plurality of alternating ridges and slots across its thickness dimension, the locus of the tips of said ridges lying along a continuously varying path whereby said ridges at the center of said electrode are farther from said slow wave structure than said ridges at the edges of said structure, and means for directing electrons in a beam along said region in energyexchanging relation with said fields of wave energy.

5. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region thereto fields of electromagnetic wave energy, an electrode spaced from and substantially coextensive with said slow wave structure, and means for directing electrons in a beam along an interaction space bounded by said structure and by said electrode in energyexchanging relation with said fields of wave energy, the spacing between said slow wave structure and said electrode continuously decreasing from the center of said electrode in directions transverse to the length and thickness of said electrode.

6. A traveling wave electron discharge device comprising a slow wave energy propagating structure producing in the region adjacent thereto fields of electromagnetic wave energy, an electrode spaced from and substantially co-extensive with said slow wave structure, said structure in said electrode bounding an interaction space, an electron source, means for producing an electric field in said interaction space, means for producing a magnetic field transverse to said electric field, electrons from said source being directed in a beam along said interaction space in energy-exchanging relation with said fields of wave energy, the spacing between said electrode and said structure continuously varying across the thickness dimension of said electrode whereby points at the center of said electrode are spaced farther from said slow Wave structure than points toward the edges of said electrode.

7. A traveling wave electron discharge device comprising a slow wave energy propagating structure, and an electrode spaced from and substantially co-extensive with said slow wave structure, a peripheral portion of said electrode facing said slow wave structure being concave as viewed from said slow wave structure, while said slow wave structure is planar as viewed from said electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,600,509 Lerbs June 17, 1952 2,681,427 Brown et al June 15, 1954 2,786,959 Warnecke et a1 Mar. 26, 1957 2,812,473 Mourier Nov. 5, 1957 2,992,354 Lerbs et a1 July 11, 1961 FOREIGN PATENTS 743,519 Great Britain Jan. 18, 1956 1,100,854 France Sept. 26, 1955 

1. A TRAVELLING WAVE ELECTRON DISCHARGE DEVICE COMPRISING A SLOW WAVE ENERGY PROPAGATING STRUCTURE PRODUCING IN THE REGION ADJACENT THERETO FIELDS OF ELECTROMAGNETIC WAVE ENERGY, AN ELECTRODE SPACED FROM AND SUBSTANTIALLY CO-EXTENSIVE WITH SAID SLOW WAVE STRUCTURE, AND MEANS FOR DIRECTING ELECTRONS IN A BEAM ALONG SAID REGION IN ENERGYEXCHANGING RELATION WITH SAID FIELDS OF WAVE ENERGY, SAID ELECTRODE HAVING A PERIPHERAL PORTION OF ITS THICKNESS DIMENSION FACING SAID SLOW WAVE STRUCTURE, AT LEAST PART OF SAID PERIPHERAL PORTION OF SAID ELECTRODE HAVING A CONTINUOUSLY VARYING SURFACE CONTOUR ACROSS ITS THICKNESS DIMENSION RESULTING IN POINTS ON SAID CONTOUR TOWARDS THE ENDS THEREOF BEING PROGRESSIVELY CLOSER TO SAID SLOW WAVE STRUCTURE. 